2 pounds skirt steak (or flank steak) For the Marinade: 1 large red onion 1 lime, juiced 1 lemon, juiced 1 tablespoon chili powder 1 tablespoon brown sugar (or honey) 1 1/2 teaspoons garlic powder (or 3 to 4 cloves garlic, minced) 1 teaspoon oregano 1/2 teaspoon ground cumin 3/4 to 1 teaspoon kosher salt 1/2 teaspoon ground black pepper 3/4 cup unsalted or low-sodium beef stock 1/2 teaspoon crushed red pepper flakes, or to taste, optional For the Pico de Gallo: 4 plum…